Fabricating thin-film optoelectronic devices with modified surface

ABSTRACT

A method ( 200 ) for fabricating thin-film optoelectronic devices ( 100 ), the method comprising: providing a substrate ( 110 ), forming a back-contact layer ( 120 ); forming at least one absorber layer ( 130 ) made of an ABC chalcogenide material, adding at least one alkali metal ( 235 ), and forming at least one cavity ( 236, 610, 612, 613 ) at the surface of the absorber layer wherein forming of said at least one cavity is by dissolving away from said surface of the absorber layer at least one crystal aggregate comprising at least one alkali crystal comprising at least one alkali metal. The method ( 200 ) is advantageous for more environmentally-friendly production of photovoltaic devices ( 100 ) on flexible substrates with high photovoltaic conversion efficiency and faster production rate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 16/144,636, filed Sep. 27, 2018, which is a divisional of U.S.patent application Ser. No. 15/312,718, now U.S. Pat. No. 10,109,761,filed Nov. 21, 2016, which is a 371 U.S. National Stage of InternationalApplication Number PCT/162015/053736, filed May 21, 2015, which claimspriority to International Application Number PCT/162014/061651, filedMay 23, 2014, the disclosures of which are all herein incorporated byreference in their entirety.

FIELD

The present invention relates to solar cells and/or optoelectronicdevices manufactured by deposition of thin-films and more particularlyto forming a modified surface of the absorber layer of optoelectronicdevices comprising chalcogenide semiconductors or ABC semiconductivecompounds.

BACKGROUND

Photovoltaic devices are generally understood as photovoltaic cells orphotovoltaic modules. Photovoltaic modules ordinarily comprise arrays ofinterconnected photovoltaic cells.

A thin-film photovoltaic or optoelectronic device is ordinarilymanufactured by depositing material layers onto a substrate. A thin-filmphotovoltaic device ordinarily comprises a substrate coated by a layerstack comprising a conductive layer stack, at least one absorber layer,optionally at least one buffer layer, and at least one transparentconductive layer stack.

The present invention is concerned with photovoltaic devices comprisingan absorber layer generally based on an ABC chalcogenide material, suchas an ABC.sub.2 chalcopyrite material, wherein A represents elements ingroup 11 of the periodic table of chemical elements as defined by theInternational Union of Pure and Applied Chemistry including Cu or Ag, Brepresents elements in group 13 of the periodic table including In, Ga,or Al, and C represents elements in group 16 of the periodic tableincluding S, Se, or Te. An example of an ABC.sub.2 material is theCu(In,Ga)Se.sub.2 semiconductor also known as CIGS. The invention alsoconcerns variations to the ordinary ternary ABC compositions, such ascopper-indium-selenide or copper-gallium-selenide, in the form ofquaternary, pentanary, or multinary materials such as compounds ofcopper-(indium, gallium)-(selenium, sulfur), copper-(indium,aluminium)-selenium, copper-(indium, aluminium)-(selenium, sulfur),copper-(zinc, tin)-selenium, copper-(zinc, tin)-(selenium, sulfur),(silver, copper)-(indium, gallium)-selenium, or (silver,copper)-(indium, gallium)-(selenium, sulfur).

The photovoltaic absorber layer of thin-film ABC or ABC.sub.2photovoltaic devices can be manufactured using a variety of methods suchas chemical vapor deposition (CVD), physical vapor deposition (PVD),spraying, sintering, sputtering, printing, ion beam, or electroplating.The most common method is based on vapor deposition or co-evaporationwithin a vacuum chamber ordinarily using multiple evaporation sources.Historically derived from alkali material diffusion using soda limeglass substrates, the effect of adding alkali metals to enhance theefficiency of thin-film ABC.sub.2 photovoltaic devices has beendescribed in much prior art (Rudmann, D. (2004) Effects of sodium ongrowth and properties of Cu(In,Ga)Se.sub.2 thin films and solar cells,Doctoral dissertation, Swiss Federal Institute of Technology. Retrieved2014 Apr. 30 from <URL:http://e-collection.ethbib.ethz.ch/eserv/eth:27376/eth-27376-02.-pdf>).

The present invention presents a method to form nanostructures, such ascavities, at the surface of a photovoltaic device's absorber layer byselectively dissolving alkali crystals embedded within the absorber'ssurface. The method advantageously uses the cavities to modify theabsorber layer surface's chemical composition, enlarge developed totalsurface, enlarge developed surface adequate for receiving dopingelements, and form point contacts with subsequently deposited thin-filmlayers.

The present invention exploits adding at least one alkali metal to athin-film optoelectronic device, and especially to its absorber layer. Apreferred at least one alkali metal comprises potassium. Adding at leastone alkali metal modifies at least the absorber layer's chemicalcontent. It may also modify the physical appearance of the surface ofthe absorber layer. Further treating of at least the surface of theabsorber layer will modify its physical appearance to revealnanostructures. Treating of absorber surface may for example be donewith a bathing apparatus. The invention discloses independent control ofseparate alkali metals during adding to layers of the optoelectronicdevice, the treating of the absorber surface, and the resulting chemicaland physical modifications to at least one absorber layer of theoptoelectronic device. Effects of the invention on at least one of thedevice's thin-film layers include at least one of doping, passivation ofabsorber surface, interfaces, grain boundaries, and defects, elementalinterdiffusion, forming of point contacts, forming of nanoholes,modification of layer roughness, optical characteristics, andoptoelectronic characteristics such as enhanced open circuit voltage andfill factor. The invention's adding of at least one alkali metal andtreating absorber surface enables manufacturing of a thinner optimalbuffer layer. In some cases a person skilled in the art mayadvantageously omit manufacturing the buffer layer. This thinner optimalbuffer layer results in reduced optical losses, thereby contributing toincrease the device's photovoltaic conversion efficiency.

SUMMARY

This invention presents a solution to the problem of manufacturing highefficiency thin-film photovoltaic or optoelectronic devices thatcomprise an ABC.sub.2 chalcopyrite absorber layer. The invention is alsoapplicable to flexible photovoltaic devices with said absorber layer. Itis also applicable to devices manufactured onto substrates, such aspolyimide, that do not comprise within the substrate alkali metals knownto augment photovoltaic conversion efficiency by diffusion into at leastthe absorber layer.

The invention presents photovoltaic (abbreviated PV) devices thatcomprise a proportionally large amount of potassium and describes thecharacteristics of said devices. The invention also presents a methodfor manufacturing said devices with the advantage of reduced opticallosses, reduced carrier recombinations in the absorber and at theinterfaces with the absorber, and therefore enhanced photovoltaicconversion efficiency. Although the method is applicable to productionon glass, metal, or various coated substrates, the method is especiallyadvantageous for the production of flexible photovoltaic devices basedon polymer substrates. Devices manufactured according to said methodhave higher photovoltaic efficiency and possibly less unwanted materialthan equivalent devices manufactured using methods described in priorart.

A common problem in the field of thin-film (abbreviated TF) PV devicesrelates to doping of the photovoltaic absorber layer for increasedefficiency. When manufactured onto glass substrates or possibly ontosubstrates coated with materials comprising alkali metals, thesubstrate's alkali metals may diffuse into the absorber layer andincrease PV conversion efficiency. In the case of substrates, such aspolyimide, that do not comprise alkali metals, the alkali-dopingelements must be supplied via deposition techniques such as, forexample, physical vapor deposition. Alkali metals may for example besupplied as a so-called post deposition treatment. The alkali metalsdiffuse during the deposition process within and across various TFlayers and their interfaces.

Another problem in the field of TF PV devices concerned with dopingrelates to efficient doping of specific areas or specific zones alongthe thin-film's thickness of the semiconductive material.

A further problem in the field of TF PV devices concerned with dopingrelates to preparing the surface, for example relates to increasing theoverall surface, made available for doping.

Yet a further problem in the field of TF PV devices relates to damage tothe CIGS that may be caused by post-CIGS deposition treatments.

A problem in the field of TF PV devices is the occurrence of carrierrecombination which results in loss of PV conversion efficiency.

Another problem in the field of TF PV devices lies at the interfacesbetween the absorber layer, the optional buffer layer, and thefront-contact layer: semiconductive junction points that desirablyinterface as point contacts with high conductivity to the front-contactlayer must be well distributed across the layer's surface. The arealdensity of said point contacts must be tuned to the thin-films'semiconductive properties.

A further problem in the field of TF PV devices is that for some bufferlayer compositions, the thicker the buffer layer, the lower its opticaltransmittance and therefore the lower the PV device's conversionefficiency.

Yet a further problem in the field of TF PV devices is that some bufferlayer compositions, such as CdS, comprise the element cadmium, thequantity of which it is desirable to minimize.

Another problem in the field of TF PV device manufacturing is that theprocess for deposition of the buffer layer, such as chemical bathdeposition (CBD), may generate waste. In the case of CdS buffer layerdeposition the waste requires special treatment and it is thereforedesirable to minimize its amount.

Yet another problem in the field of flexible TF PV device manufacturingis that it is desirable to benefit from large process windows formaterial deposition, and more specifically in relation to thisinvention, the process window for the adding of alkali metals andsubsequent deposition of at least one buffer layer.

Finally, a problem in the field of TF PV devices is that of the color ofthe device. This problem may be even more important in the context ofassemblies of PV devices, such as large PV modules, where multipledevices are placed next to each other and a desired match between thecolor of devices is desired. This may be for example to manufactureassemblies of PV devices of uniform color. It may also be to manufacturePV assemblies where different colors among PV devices are used to designpatterns, writings, or gradients.

Briefly, the invention thus pertains to a method of fabricating TF PVdevices comprising at least one ABC.sub.2 chalcopyrite absorber layer,to adding at least one alkali metal, thereby forming alkali crystalsembedded in the surface of the absorber layer, to selectively dissolvingsaid alkali crystals, thereby leaving cavities at the surface of theabsorber layer, and to treating the absorber layer surface. Theresulting TF PV devices comprise at least one ABC.sub.2 chalcopyriteabsorber layer that may be characterized as having at its surface alarge density of cavities of nanoscopic scale.

For the purposes of the present invention, the term “adding” or “added”refers to the process in which chemical elements, in the form ofindividual or compound chemical elements, namely alkali metals and theirso-called precursors, are being provided in the steps for fabricatingthe layer stack of an optoelectronic device for any of: [0023] forming asolid deposit where at least some of the provided chemical elements willdiffuse into at least one layer of said layer stack, or [0024]simultaneously providing chemical elements to other chemical elementsbeing deposited, thereby forming a layer that incorporates at least someof the provided chemical elements and the other elements, or [0025]depositing chemical elements onto a layer or layer stack, therebycontributing via diffusion at least some of the provided chemicalelements to said layer or layer stack.

In greater detail, the method for fabricating thin-film optoelectronicdevices comprises providing a substrate, forming a back-contact layer,forming at least one absorber layer, which absorber layer is made of anABC chalcogenide material, including ABC chalcogenide material ternary,quaternary, pentanary, or multinary variations, wherein A representselements of group 11 of the periodic table of chemical elements asdefined by the International Union of Pure and Applied Chemistryincluding Cu and Ag, B represents elements in group 13 of the periodictable including In, Ga, and Al, and C represents elements in group 16 ofthe periodic table including S, Se, and Te, adding at least one alkalimetal, and forming at least one cavity at the surface of the absorberlayer, wherein forming of said at least one cavity is by dissolving awayfrom said surface of the absorber layer at least one crystal aggregatecomprising at least one alkali crystal comprising at least one alkalimetal.

In said method, the at least one alkali metal may comprise potassium.Furthermore, the method may comprise a step of aqueous wetting of atleast the surface of the absorber layer with a diluted aqueous ammoniasolution with a diluted ammonia molarity in the range from 0 to 20 M,preferably in the range from about 1 M to 10 M, more preferably in therange from about 2 M to 4 M. The method may comprise a step of treatingthe absorber layer surface by adding oxidation state +1/+2 elements tothe surface of the absorber layer. At least one absorber layer may beCu(In,Ga)Se.sub.2. The method may also comprise forming at least onefront-contact layer. Furthermore, the reflectance of the thin-filmoptoelectronic device may be adjusted by adjusting the amount of atleast one alkali metal in the step of adding at least one alkali metal.The substrate may be delivered between a delivery roll and a take-uproll of a roll-to-roll manufacturing apparatus.

The invention also pertains to a TF optoelectronic device obtainable bythe described method, comprising: a thin-film optoelectronic deviceobtainable by the aforementioned method, comprising: a substrate; aback-contact layer; at least one absorber layer, which absorber layer ismade of an ABC chalcogenide material, including ABC chalcogenidematerial quaternary, pentanary, or multinary variations, wherein Arepresents elements of group 11 of the periodic table of chemicalelements as defined by the International Union of Pure and AppliedChemistry including Cu and Ag, B represents elements in group 13 of theperiodic table including In, Ga, and Al, and C represents elements ingroup 16 of the periodic table including S, Se, and Te; at least onealkali metal; and at least one cavity at the surface of the absorberlayer; wherein the form of said at least one cavity is the result ofdissolving away from said surface of the absorber layer at least onecrystal aggregate comprising at least one alkali crystal comprising atleast one alkali metal. In said device, the at least one alkali metalmay comprise potassium. Also, the at least one alkali crystal maycomprise a cubic crystal. Furthermore, the at least one alkali metal maycomprise potassium and wherein a curve for counts of potassium in thedevice's sputter profiling graph comprises an upper peak of potassiumwithin a depth ranging from the surface of the absorber layer to about0.5 .mu.m into the absorber layer. Said upper peak of potassium maycomprise a base, the width of which is in the range from about 0.1 .mu.mto 0.5 .mu.m, preferably from about 0.15 .mu.m to 0.3 .mu.m, morepreferably about 0.2 .mu.m. Said upper peak of potassium may have aheight that, measured from the number of counts above its base, is inthe range from about 0.2 to 17 times, preferably from about 0.6 to 6times, more preferably from about 1.1 to 2 times the number of countsfrom the point of minimum number of counts in potassium of the absorberlayer to the number of counts at the base of said upper peak ofpotassium. Furthermore in the device, the at least one alkali metal maycomprises potassium and the curve for the copper to selenium content inthe device's energy dispersive X-ray line scan graph may comprise anextended region of low and about constant Cu/Se extending from thesurface of the absorber layer into a portion of the depth of theabsorber layer, said low and about constant region has a depth in therange from about 0.05 .mu.m to 0.5 .mu.m, preferably about 0.1 .mu.m to0.4 .mu.m, more preferably about 0.2 .mu.m to 0.3 .mu.m. Also in thedevice, the at least one alkali metal may comprise potassium, comprise+1/+2 elements, and the X-ray photoelectron spectrometry curve maycomprise a Cd 3d5/2 peak that is at least about 60% greater in height,preferably in the range of about 60% to 1000% greater in height, morepreferably in the range of about 380% to 530% greater in height, thanfor the curve of a device wherein the at least one alkali metal does notcomprise potassium. Furthermore, the surface of the absorber layer maycomprises a plurality of cavities, said cavities covering a totalrelative projected area in the range from about 15% to 80%, preferablyabout 20% to 60%, more preferably about 25% to 45%. In greater detail,said cavities may have a mean cavity area in the range from about0.1.times.10.sup.-15 m.sup.2 to about 0.8.times.10.sup.-15 m.sup.2,preferably from about 0.2.times.10.sup.-15 m.sup.2 to about0.6.times.10.sup.-15 m.sup.2, more preferably from about0.3.times.10.sup.-15 m.sup.2 to about 0.5.times.10.sup.-15 m.sup.2. Atleast one cavity may comprise cadmium. Said device may also comprise atleast one front-contact layer.

ADVANTAGES

A main advantage of the invention is that it may enable, in a singlemanufacturing step with reduced and more efficient consumption ofchemical products, the fabrication of a photovoltaic device withmodified CIGS layer surface by contributing the features of surfacenanostructuring, reduced surface damage, doping, buried and discretesemiconductive junction formation, and formation of point contacts. Theresulting device may also feature reduced or no cadmium content.

Advantages of the invention derive from a method of adding substantialamounts of alkali elements to the absorber layer, selectively dissolvingalkali crystals at the surface of the absorber layer, and addingoxidation state +1 and/or +2 elements to the absorber layer. Highefficiency PV devices resulting from the method may be advantageous,thanks to a thinner or an absence of buffer layer, over prior artdevices where little or no alkali elements have been added. The methodmay also be advantageous over prior art devices where no nanocavitieshave been formed. An advantageous effect of the invention is that theoptimal thickness for an optional buffer layer coating said absorberlayer may be thinner than the optimal buffer layer needed for prior artPV devices with comparable PV efficiency. The invention may shorten themanufacturing process, reduce environmental impact of manufacturing andof the resulting device, and increase device PV conversion efficiency.

The invention's features may advantageously solve several problems inthe field of TF PV devices manufacturing, and more specificallymanufacturing of the absorber and buffer layer of such devices. Thelisted advantages should not be considered as necessary for use of theinvention. For manufacturing of TF flexible PV devices manufactured tothe present invention, the advantages obtainable over devices and theirmanufacturing according to prior art include: higher PV conversionefficiency, improved absorber doping, especially towards the part of thelayer that is closest to the front-contact layer, absorber layernanostructuring that improves dopant effect and contributes to theformation of point contacts with the front-contact layer, improvedsemiconductor junction that helps reduce carrier recombinations, reducedsemiconductor bulk defects that help reduce carrier recombinations,thinner or omitted buffer layer, shorter buffer layer deposition time,enlarged buffer layer deposition process window, enlarged depositionprocess window for alkali metal doping elements, increase inopen-circuit voltage resulting from a decrease in recombination-relevantsurface, enlarged tuning window for (in, Ga) addition to the CIGSabsorber layer, improved copper to cadmium interface resulting in lowercadmium content in device, facilitated growth of stoichiometric absorbermaterial, more environmentally-friendly manufacturing process anddevices, lower manufacturing costs, adjustment of PV device color and/orreflectance

BRIEF DESCRIPTION OF FIGURES

Embodiments of the invention will now be described by way of example,with reference to the accompanying drawings, in which:

FIGS. 1A and 1B are cross-sections of an embodiment of a thin-filmoptoelectronic device.

FIG. 2 presents steps in a method to manufacture a thin-filmoptoelectronic device.

FIGS. 3A-3B are sputter depth profiling graphs obtained using second ionmass spectrometry (SIMS).

FIG. 3C compares the Cu/Se compositional profile of absorbercross-sections measured by energy-dispersive X-ray spectroscopy (EDX) ina transmission electron microscope (TEM).

FIGS. 4A-4D are graphs enabling comparison of process parameters forforming CIGS surface cavities.

FIGS. 5A-5E are graphs derived from X-ray photoelectron spectrometry(XPS) enabling comparison of the effect of adding at least one alkalimetal on the chemical composition of the absorber layer surface.

FIGS. 6A-6F are electron microscopy images of absorber surface regionsshowing examples of cavities and indications for the method used toevaluate their distribution.

FIG. 7 is a graph of device reflectance as a function of wavelengthprior to forming a front-contact layer.

FIG. 8 is a schematic side view for an apparatus for carrying out themethod of fabricating thin-film optoelectronic devices.

DETAILED DESCRIPTION

In simplified terms, the following description details a TF PV deviceand especially the surface of its absorber layer which comprises aplurality of cavities formed by selectively dissolving away alkalicrystals embedded in the absorber's surface. The cavities result fromthe dissolution of individual crystals and/or aggregates of crystals.The shape of the cavities, or nanocavities, corresponds to the geometryof the dissolved alkali crystals and their size may range from a few toover a hundred nanometers. Although the shape of the cavities moregenerally derives from cubic or aggregates of cubic crystals, the shapemay also be rectangular, tetragonal, pyramidal, and possibly circular,or elliptic. The shape of cavities is a function of step temperaturesand step durations used in the manufacturing method. The surface of theabsorber layer is also doped. The description also details the method tomanufacture the TF PV device, parameters to grow the alkali crystalsthat are embedded into the surface of the absorber layer, how toselectively dissolve said crystals without damaging the absorber layer,and how to dope the surface of the absorber layer within the overallmanufacturing process. In effect, by forming cavities, the method mayincrease the total surface available to dope the surface of the absorberlayer and at the same time reduce the volume within the absorber layerwhere detrimental carrier recombination may occur.

The invention is applicable to substrates that may or substrates thatmay not diffuse alkali elements into thin-film layers. For example themethod is applicable to potassium-nondiffusing substrates. A“potassium-nondiffusing substrate” is a component, ordinarily a sheet ofmaterial, that comprises no potassium or so little potassium thatdiffusion of potassium elements into the subsequently described layersis considered too small to significantly alter the optoelectronicproperties of the device. Potassium-nondiffusing substrates also includesubstrates that comprise means to prevent diffusion of potassium intocoatings or layers supported by the substrate. A potassium-nondiffusingsubstrate may for example be a substrate that has been specially treatedor coated with a barrier layer to prevent diffusion of potassiumelements into coatings or layers supported by the substrate. Speciallytreated substrates or barrier-coated substrates ordinarily prevent thediffusion of a broad range of elements, including alkali metals, intocoatings or layers supported by the substrate.

For clarity, components in figures showing embodiments are not drawn atthe same scale.

FIG. 1A presents the cross-section of an embodiment of a TFoptoelectronic or PV device 100 comprising a substrate 110 for a stackof material layers.

Substrate 110 may be rigid or flexible and be of a variety of materialsor coated materials such as glass, coated metal, polymer-coated metal,polymer, coated polymer such as metal-coated polymer, or flexible glass.A preferred flexible substrate material is polyimide as it is veryflexible, sustains temperatures required to manufacture high efficiencyoptoelectronic devices, requires less processing than metal substrates,and exhibits thermal expansion coefficients that are compatible withthose of material layers deposited upon it. Industrially availablepolyimide substrates are ordinarily available in thicknesses rangingfrom 7 .mu.m to 150 .mu.m. Substrate 110 may be a potassium-nondiffusingsubstrate. Polyimide substrates are ordinarily considered aspotassium-nondiffusing.

At least one electrically conductive layer 120 coats substrate 110. Saidelectrically conductive layer, or stack of electrically conductivelayers, also known as the back-contact, may be of a variety ofelectrically conductive materials, preferably having a coefficient ofthermal expansion (CTE) that is close both to that of the said substrate110 onto which it is deposited and to that of other materials that areto be subsequently deposited upon it. Conductive layer 120 preferablyhas a high optical reflectance and is commonly made of Mo althoughseveral other TF materials such as metal chalcogenides, molybdenumchalcogenides, molybdenum selenides (such as MoSe.sub.2), Na-doped Mo,K-doped Mo, Na- and K-doped Mo, transition metal chalcogenides, dopedindium oxides, for example tin-doped indium oxide (ITO), non-dopedindium oxides, doped or non-doped zinc oxides, zirconium nitrides, tinoxides, titanium nitrides, Ti, W, Ta, Au, Ag, Cu, and Nb may also beused or included advantageously.

At least one absorber layer 130 coats electrically conductive layer 120.Absorber layer 130 is made of an ABC material, wherein A representselements in group 11 of the periodic table of chemical elements asdefined by the International Union of Pure and Applied Chemistryincluding Cu or Ag, B represents elements in group 13 of the periodictable including In, Ga, or Al, and C represents elements in group 16 ofthe periodic table including S, Se, or Te. An example of an ABC.sub.2material is the Cu(In,Ga)Se.sub.2 semiconductor also known as CIGS.

Optionally, at least one semiconductive buffer layer 140 coats absorberlayer 130. Said buffer layer ordinarily has an energy bandgap higherthan 1.5 eV and is for example made of CdS, Cd(S,OH), CdZnS, indiumsulfides, zinc sulfides, gallium selenides, indium selenides, compoundsof (indium, gallium)-sulfur, compounds of (indium, gallium)-selenium,tin oxides, zinc oxides, Zn(Mg,O)S, Zn(O,S) material, or variationsthereof.

At least one transparent conductive layer 150 coats buffer layer 140.Said transparent conductive layer, also known as the front-contact,ordinarily comprises a transparent conductive oxide (TCO) layer, forexample made of doped or non-doped variations of materials such asindium oxides, tin oxides, or zinc oxides.

Contributing to this invention, the amount of potassium comprised in theinterval of layers 370 from electrically conductive back-contact layer120, exclusive, to transparent conductive front-contact layer 150,inclusive, is in the range between 500 and 10000 potassium atoms permillion atoms (ppm). A TF PV device demonstrating superior PV conversionefficiency preferably has an amount of potassium comprised in saidinterval of layers 370 in the range between 1000 and 2000 potassiumatoms per million atoms. For a device comprising at least two alkalimetals, one of which is potassium, the amount of said at least onealkali metal other than potassium may be in the range of 5 to 5000 ppm.the amount of at least one alkali metal other than potassium is at most½ and at least 1/2000 of the comprised amount of potassium.

Optionally, front-contact metallized grid patterns 160 may cover part oftransparent conductive layer 150 to advantageously augment front-contactconductivity. Also optionally, said TF PV device may be coated with atleast one anti-reflective coating such as a thin material layer or anencapsulating film.

FIG. 1B presents a detail of a cross-section of an embodiment of a TFoptoelectronic or PV device 100. FIG. 1B shows part of absorber layer130 and part of optional buffer layer 140. The place of buffer layer 140may be replaced by other types of transparent layers, for example atransparent conductive layer 150. FIG. 1B also shows, near and/or at thesurface of absorber layer 130, a plurality of cavities 610, 612, 613.Said cavities result from selectively dissolving alkali crystals oraggregates of alkali crystals embedded within the surface of theabsorber layer without dissolving the ABC.sub.2 material of the absorberlayer. The shape of said cavities in the cross-section ordinarilyresemble squares, rectangles, and aggregates of squares and rectangles.The shape of said cavities may also resemble crystals subject to crystaltwinning. FIG. 1B illustrates sixteen of said cavities 610, 612, 613.The surface of absorber layer 130 also comprises at least one oxidationstate +1 element and/or at least one oxidation state +2 element (seedescription for FIG. 2). On the cross-section of the TF device, thesurface of the absorber layer 130 is defined as the region in which thecavities 610, 612, 613 extend into the absorber layer and additionallyup to about 80 nm, preferably 50 nm, into the absorber layer 130.Consequently, absorber layer 130 is mostly a p-type absorber but, at itssurface in contact with at least one of buffer layer 140 orfront-contact layer 150, comprises n-type absorber material. Absorberlayer 130 may therefore comprise at least one homojunction, preferably aplurality of homojunctions. Said homojunctions may be located at saidcavities, preferably within the absorber layer material surrounding saidcavities comprised in a radius of about 100 nm. An example ofhomojunction for an ABC.sub.2 absorber layer is a p-type CIGS absorberlayer comprising n-type CIGS at the surface. Furthermore, the surface ofthe absorber layer may also comprise at least one heterojunction,preferably a plurality of heterojunctions. Said heterojunction may forexample comprise a p-type CIGS absorber layer combined with n-typematerial such as CdS, Cd(S,OH), Cd(O,H), CdZnS, KInSe.sub.2, indiumsulfides, zinc sulfides, gallium selenides, indium selenides, compoundsof (indium, gallium)-sulfur, compounds of (indium, gallium)-selenium,tin oxides, zinc oxides, Zn(Mg,O)S, Zn(O,S) material, or variationsthereof.

FIG. 2 presents a method 200 comprising material deposition steps tomanufacture said TF optoelectronic or PV device 100 comprising a stackof material layers where at least one alkali metal, one of themcomprising potassium, has been added. The method is considered to beespecially appropriate for substrates considered potassium-nondiffusingor that may comprise at least one barrier layer that prevents thediffusion of alkali metals from the substrate into subsequentlydeposited coatings. The method as described is applicable to productionon glass, metal, or various coated substrates, and is especiallyadvantageous for polymer substrate materials such as polyimide.

An exemplary sequence of material layer deposition and treatmentfollows. The purpose of this description is to clarify the contextwithin which treatment after deposition of the absorber layer, the mainsubject of this invention, occurs. Between any of the subsequentmanufacturing steps, a person skilled in the art will know how to, as anoption, temporarily store the unfinished product, for example within avacuum or near vacuum container or even within a container characterizedby a controlled atmospheric environment or possibly a containercomprising at least one inert gas. Exposure to air and/or to humidity isknown to have an effect on the chemical composition of the materiallayers deposited during the material deposition steps. A person skilledin the art will advantageously use steps of the method within amanufacturing sequence where exposure to air and/or humidity is kept toa minimum between steps. A person skilled in the art may also use stepsof the method so that the intermediate product is only exposed to acontrolled environment between steps, such as vacuum, near vacuum, lowhumidity atmosphere, or at least one inert gas. In case of exposure toair between steps that is different from air at standard ambienttemperature and pressure (SATP) and 40% relative humidity (RH), a personskilled in the art will adapt the limits for said cumulated minutes inair according to changes in aforementioned environmental parameters. Inthe rest of this document, the word minutes is abbreviated as min.

The method starts at step 210 by providing a substrate. Said substratemay be a potassium-nondiffusing substrate.

Following step 210 and prior to the step of forming cavities 236, addingof at least one alkali metal 235 occurs as at least one event duringand/or between any of steps comprised in the interval from step ofproviding substrate 210 (excluding the step 210 itself), to the step offorming cavities 236 (excluding the step 236 itself). Said at least oneof said alkali metal preferably comprises potassium. The fact that theadding may occur during or between said interval of steps is representedby dashed arrows emanating from block 235 in FIG. 2. Each of said alkalimetals may be added simultaneously with any of the other of said alkalimetals and/or during separate adding events. Adding of each of saidalkali metals may comprise any or a combination of adding a layer orprecursor layer of at least one of the alkali metals, co-adding at leastone of the alkali metals with the forming of any of the method'smaterial layers, or diffusing at least one of the alkali metals from atleast one layer into at least one other material layer. Preferably,adding of at least one different alkali metal is done in the presence ofat least one said C element. More preferably, adding of potassium, forexample by adding via a so-called potassium-comprising precursor such asKF, KCl, KBr, KI, K.sub.2S, K.sub.2Se, is done in the presence of atleast one said C element.

At step 220, forming at least one back-contact layer comprisesdepositing at least one electrically conductive layer. Forming of theback-contact layer may be done using a process such as sputtering,spraying, sintering, electrodeposition, CVD, PVD, electron beamevaporation, or spraying of the materials listed in the description ofsaid electrically conductive layer 120.

At step 230, forming at least one absorber layer comprises coating saidelectrically conductive layer with at least one ABC absorber layer 130.The materials used correspond to those in the description provided forABC absorber layer 130. Said absorber layer may be deposited using avariety of techniques such as sputtering, spraying, sintering, CVD,electrodeposition, printing, or as a preferred technique for an ABCmaterial, physical vapor deposition. Substrate temperatures duringabsorber layer deposition are ordinarily comprised between 100.degree.C. and 650.degree. C. The range of temperatures and temperature changeprofiles depend on several parameters including at least the substrate'smaterial properties, the supply rates of the materials that compose theABC material, and the type of coating process. For example, for a vapordeposition process, substrate temperatures during forming of theabsorber layer will ordinarily be below 600.degree. C., and if usingsubstrates requiring lower temperatures, such as a polyimide substrate,preferably below 500.degree. C., and more preferably in the range from100.degree. C. to 500.degree. C. For a co-evaporation vapor depositionprocess, substrate temperatures during forming of the absorber layerwill ordinarily be in the range from 100.degree. C. to 500.degree. C.Said substrate temperatures may be advantageously used with a polyimidesubstrate.

For a deposition process such as physical vapor deposition, for exampleif forming absorber layer 230 is done using a physical vapor depositionprocess, adding of potassium as part of adding at least one alkali metal235 may be done during and/or in continuation of the physical vapordeposition process by supplying potassium fluoride, KF. This may, forexample, be advantageous when manufacturing with a co-evaporationphysical vapor deposition system. Adding the alkali metal potassium willpreferably be done in the presence of a flux of element Se supplied at arate in the range of 5 to 100 .ANG./s, preferably at a rate in the rangeof 20 to 50 .ANG./s.

Substrate temperature ranges for said adding of at least one alkalimetal are from 100.degree. C. to 700.degree. C., preferably from250.degree. C. to 450.degree. C., more preferably from 330.degree. C. to370.degree. C. A person skilled in the art will select appropriatetemperatures for said adding of at least one alkali metal so that theyare compatible with the materials deposited, TF properties, andsubstrate. For example, one skilled in the art of physical vapordeposition processes will know that potassium, for example in the formof KF, may be added at higher temperatures than some other alkali metalssuch as sodium, for example in the form of NaF. The possibility ofhigher adding temperature for KF may advantageously be used to addalkali metals starting with potassium at temperatures closer to thoseused at step 230 and, as the substrate temperature decreases, tocontinue with adding of same and/or other alkali metals. For example,adding of at least one alkali metal may be with a physical vapordeposition process where alkali metal potassium, for example in the formof a KF potassium-comprising precursor, is supplied at a rate equivalentto an effective layer deposition of about 1 nm/min to 2 nm/min for aduration of 20 minutes. For another example, adding of at least onealkali metal preferably uses a physical vapor deposition process wheresodium, for example in the form of NaF sodium-comprising precursor, isfirst added at a rate of about 1 nm/min to 2 nm/min for a duration of 20minutes and followed, possibly as part of a co-evaporation process, byadding of potassium, for example in the form of a KFpotassium-comprising precursor, at a rate of about 1 nm/min to 2 nm/minfor a duration of 20 minutes. A person skilled in the art may adaptdeposition rate and duration. The skilled person will also know thatadding of at least one alkali metal may take place with adding of one ormore of said at least one alkali metal at substrate temperaturesordinarily lower than 700.degree. C. and possibly much lower than350.degree. C., such as at ambient temperatures of about 25.degree. C.and below. The substrate may then be heated afterwards, therebyfacilitating diffusing of said alkali metals to the TF layers of theoptoelectronic device, possibly in combination with depositing at leastone C element. Adding at least one alkali metal 235 is preferably donein the presence of Se.

The amount of potassium added by adding at least one alkali metal 235 issuch that following forming of front-contact layer 150 at later step250, said amount comprised in the interval of layers 370 fromback-contact layer 120, exclusive, to front-contact layer 150,inclusive, is in the range between 500 and 10000 potassium atoms permillion atoms and the amount of the other of said at least one alkalimetal is in the range of 5 to 5000 ppm and at most ½ and at least 1/2000of the comprised amount of potassium. A TF PV device that has a superiorPV conversion efficiency preferably has an amount comprised in saidinterval of layers 370 from about 1000 to 2000 potassium atoms permillion atoms.

Also, adding some of at least one alkali metal preferably occurs afterforming the last of at least one absorber layer 230 and before at leastone of step of forming cavities 236. The latter adding some of at leastone alkali metal preferably comprises an alkali compound that comprisespotassium. Said alkali compound may comprise any of an alkali metal, analkali halide, or an alkali salt.

At step 236, that of forming cavities, at least the surface of absorberlayer 130 is subject to at least one of the steps of aqueous wetting237, treating absorber surface 238, and forming buffer layer 240. Atstep 236 at least one, preferably a plurality, of alkali crystalsembedded within the surface of the absorber layer is dissolved. Part ofthe absorber layer material directly underlying at least one of saidalkali crystal may also be dissolved, said part will ordinarily notextend deeper into the absorber than one height unit corresponding tothe depth of the crystal embedded into the absorber layer. Also, part ofthe surface of the absorber layer directly in contact with said alkalicrystals may be dissolved. Furthermore, part of the surface of theabsorber layer may also be dissolved.

At optional step 237, represented as a dashed box because the step maybe considered optional, aqueous wetting comprises wetting at least thesurface of absorber layer 130 with at least one aqueous wetting. Aqueouswetting will preferably be done in a bath. Aqueous wetting may also bedone using spraying. Aqueous wetting is preferably with a dilutedaqueous ammonia solution. Exposure to air is known to have an effect onthe chemical composition of the absorber layer. A person skilled in theart will try to minimize the duration of exposure of the device to air.Although the duration of exposure may be of several days, aqueouswetting preferably occurs after the substrate has spent a duration of atmost 10, more preferably less than 5, cumulated minutes in air at SATPand 40% RH after completion of the forming absorber layer step 230.Described briefly, composition of the preferred diluted aqueous ammoniasolution bath is an aqueous solution with a molarity in the range from 3M to 5 M. Said diluted aqueous ammonia solution comprises a mixture ofwater and commercially available ammonia aqueous solution. Parametersrelevant to the aqueous wetting step comprising a bath comprise acumulated duration of about 10 min, preferably, about 2 min, a bathtemperature of about 25.degree. C. and a molarity of about 3 mol/L (alsowritten M). Ranges, preferred ranges, and most preferred ranges for saidparameters are presented in Table 1. Most preferred ranges are at leastapplicable using a bath. A person skilled in the art will readily adaptparameters to other types of wetting apparatuses.

Wetting steps may take place in a chemical bath deposition (CBD)apparatus and/or a spraying apparatus. A chemical bath deposition systemensures continuous flow and mixing of the bathing solution over at leastabsorber layer 130.

At steps treating absorber surface 238 and/or forming buffer layer 240,so-called “oxidation state +1” and/or so-called “oxidation state +2”elements, thereafter abbreviated +1/+2 elements, are added to theabsorber layer 130 and especially to the surface of the absorber layer.Adding of oxidation state +1/+2 elements to the surface of the absorberlayer transforms at least at portion of the surface from a p-typeabsorber surface into an n-type absorber surface, thereby forming aburied junction at least a portion of the absorber layer. Said buriedjunction may be a homojunction. Said buried junction is preferably a p-njunction. Resulting presence of oxidation state +1/+2 elements into theabsorber layer may then, for example, be the result of physisorption,chemisorption, or diffusion. Said oxidation state +1/+2 elementscomprise at least one element of at least one of group 2, group 3,lanthanide series, actinide series, or transition metals from theperiodic table of elements. A commonly used oxidation state +1/+2element is cadmium. Although this description focuses mostly onsolutions comprising cadmium, a person skilled in the art may adapt theinvention to use steps that comprise other or additional oxidation state+1/+2 elements. For example, a person skilled in the art may want toreduce or eliminate the amount of cadmium comprised in the resulting PVdevice by replacing part or all of the cadmium used in the method withat least one other oxidation state +1/+2 element.

At step 238, represented as a dashed box because the step may beconsidered optional, at least absorber layer 130 is subject to at leastone treating absorber surface 238. The step of treating absorber layer238 is especially useful if the step of forming buffer layer 240 isomitted. The duration of exposure to air between aqueous wetting 237 andtreating absorber surface 238 is preferably at most 2, preferably lessthan 0.5, cumulated minutes in air at aforementioned SATP and RH of step237. Described briefly, relevant parameters for treating absorbersurface comprise treating for a cumulated duration of about 22 minutesat least absorber layer 130 into a solution with temperature of about70.degree. C. comprising, per liter of water, an amount of about 60 mLof cadmium (Cd) solution and an amount of about 140 mL of ammonia(NH.sub.3) solution. Ranges for said parameters are presented inTable 1. Said water is preferably distilled water or deionized water,more preferably ultra-pure water with a resistivity of about 18M.OMEGA.cm. Said cadmium solution comprises a Cd salt solution with amolarity most preferably in the range from about 0.026 to 0.03 mol/L.Concentration of said ammonia solution is more preferably in the rangefrom 14.3 to 14.7 M. Ranges, preferred ranges, and most preferred rangesfor said parameters are presented in Table 1. Most preferred ranges areat least applicable using a bathing apparatus.

TABLE 1 Ranges for steps 237 to 246. Range Pref. range More pref. rangefrom-to from-to from-to Step 237: Aqueous wetting Duration [min]0.05-7200  0.1-30  1-5 Temperature [° C.] 2-95 10-50 23-27 Dilutedammonia molarity [mol/L] 0-20  1-10 2-4 Step 238: Treating absorbersurface Duration [min] 0.05-7200  0.1-30  20-24 Temperature [° C.] 2-9510-85 65-75 Amount of Cd solution per liter [mL]  10-1000  20-100 55-65Amount of NH₃ solution per liter [mL]  0-990  20-400 130-150 Cd molarityin Cd solution [mol/L] 0.00001-10     0.01-0.1  0.026-0.03  Ammoniasolution molarity [mol/L] 1-30 10-20 14.3-14.7 Step 240: Forming bufferlayer Duration [min] 0.05-7200  0.1-30  14-18 Temperature [° C.] 2-9510-85 65-75 Cd salt solution molarity [mol/L] 0.00001-10     0.01-0.1 0.026-0.030 Ammonia solution molarity [mol/L] 1-30 10-20 14.3-14.7Thiourea aq. sol. molarity [mol/L] .01-10   0.2-0.6 0.35-0.4  Step 244:Annealing Cadmium sulfide, temperature [° C.] 100-300  150-200 175-185Cadmium sulfide, duration [min] 1-30 1-5 1.8-2.2 Zinc oxi-sulfide,temperature [° C.] 100-300  150-250 190-210 Zinc oxi-sulfide, duration[min] 1-30  5-15  9-11 Step 246: Degassing Degassing temperature [° C.]10-150 20-60 28-32 Degassing duration [min]  0.1-3000 1440-28801800-2400

At optional step 240, represented as a dashed box because the step maybe considered optional, forming buffer layer comprises coating saidabsorber layer with at least one so-called semiconductive buffer layer140. The materials used correspond to those in the description providedfor buffer layer 140. Said buffer layer may be deposited using a varietyof techniques such as CVD, PVD, sputtering, sintering,electrodeposition, printing, atomic layer deposition, or as a well knowntechnique at atmospheric pressure, chemical bath deposition (CBD). Toform a cadmium sulfide buffer layer, a person skilled in the art mayform at least one buffer layer bath for CBD comprising a Cd salt aqueoussolution of about 0.028 M concentration and an ammonia solution of about14.5 M concentration that are first mixed together with water,preferably high-purity water (18 M.OMEGA.cm), in a volume ratio of about3:7:37, preheated to about 70.degree. C. for about 2 min, and thensupplemented with a thiourea aqueous solution of about 0.374 Mconcentration. At least absorber layer 130 is then immersed within saidbuffer layer bath that is maintained to a temperature of about70.degree. C. until the desired buffer layer thickness is obtained. Atleast absorber layer 130 is then washed with water, preferablyhigh-purity water.

Following at least one step of forming cavities 236, a person skilled inthe art will notice, for example using TF analysis techniques such asX-ray photoelectron spectrometry (XPS) or secondary ion massspectrometry (SIMS), that the surface of the absorber layer comprisesmore oxidation state +1/+2 elements than the surface of absorber layersmanufactured according to prior art or, for an analysis usinginductively coupled plasma mass spectrometry (ICP-MS), that theconcentration of the +1/+2 elements in the absorber layer is higher thanin absorber layers manufactured according to prior art.

For example, for devices where the oxidation state +1/+2 element(s)added at the step of forming cavities 236 is cadmium, an ICP-MS analysisof the resulting device after forming cavities is summarized in Table 2.Here, adding at least one alkali metal 235 is provided by evaporatingmaterial comprising potassium fluoride (KF) to add potassium to theabsorber layer 130. Three devices are compared: a first device where nopotassium is added, a second, best mode device, subjected to about 20minutes of evaporation, and a third device subjected to about 60 minutesof evaporation. The evaporation rates are those mentioned previously.Evaporations flux rates and corresponding evaporation durations will beadapted by the person skilled in the art. A person skilled in the artmay therefore advantageously use a manufacturing method where adjustingthe amount of adding at least one alkali metal 235, or for a morespecific example, adding potassium, contributes to regulating the amountof oxidation state +1/+2 elements that are added to or absorbed by theabsorber layer. Said method of adding at least one alkali metal 235 mayalso contribute to adjusting the duration of the step of formingcavities 236 or steps therein 237, 238, 240.

TABLE 2 Effect on cadmium concentration from post-deposition addingpotassium Cadmium concentration (atomic percentage) Range Pref. rangeMost pref. range from-to from-to from-to Device with no 0.02-0.150.05-0.1  0.07-0.08 added potassium Dev. with 20 min. 0.05-1   0.1-0.40.2-0.3 added potassium Dev. with 60 min. 0.1-2   0.2-0.8 0.4-0.5 addedpotassium

Steps 238 and/or 240 are preferably done in at least one bathingapparatus. However, a person skilled in the art will readily adapt themethod for use with a spraying apparatus. It is also possible to usephysical vapor deposition (PVD) apparatuses and a plurality ofassociated deposition methods, the most common being evaporativedeposition or sputter deposition. A person skilled in the art willreadily manufacture a set of devices covering a range of parameters, forexample a range of durations for at least one of steps 237, 238, 240,244, 246, so as to obtain a photovoltaic device where, for example,photovoltaic efficiency is maximized. Other objectives may be, forexample, to minimize buffer layer thickness, maximize efficiency tobuffer layer thickness ratio, or maximize efficiency to cadmium contentratio.

At optional step 242 at least absorber layer 130 is subject to at leastone short drying step. The short drying step may be done in air,preferably using a blown inert gas, more preferably using at least oneionization blow off nozzle, even more preferably with the blown inertgas being nitrogen.

At optional step 244 at least absorber layer 130 is subject to at leastone annealing step. For a cadmium sulfide buffer layer, said annealingstep is preferably done at about 180.degree. C., preferably in air forabout 2 min. For a zinc oxi-sulfide buffer layer, said annealing step ispreferably done at about 200.degree. C., preferably in air for about 10min.

At optional step 246, at least absorber layer 130 is subject to at leastone degassing step. This step is ordinarily not needed for devices wherethe substrate does not absorb humidity, for example glass or metalsubstrates. Said degassing is preferably done in vacuum. The degassingstep is preferred for substrate materials, for example polyimide, thatmay have absorbed humidity at previous manufacturing steps. For example,for a degassing temperature of about 25.degree. C., an effectivedegassing duration is about 35 hours.

Parameter ranges for steps 237 to 246 are provided in Table 1. To tunethe optional process of forming the buffer layer of step 240, oneskilled in the art will ordinarily develop a test suite over a range ofbuffer coating process durations to manufacture a range of PV devicescomprising a range of buffer layer thicknesses. One will then select thebuffer coating process duration that results in highest PV deviceefficiency.

The following steps describe how to complete the manufacture of aworking PV device benefiting of the invention.

At step 250, forming front-contact layer comprises coating said bufferlayer with at least one transparent conductive front-contact layer 150.Said front-contact layer ordinarily comprises a transparent conductiveoxide (TCO) layer, for example made of doped or non-doped variations ofmaterials such as indium oxide, gallium oxide, tin oxide, or zinc oxidethat may be coated using a variety of techniques such as PVD, CVD,sputtering, spraying, CBD, electrodeposition, or atomic layerdeposition.

At optional step 260, forming front-contact grid comprises depositingfront-contact metallized grid traces 160 onto part of transparentconductive layer 150. Also optionally, said TF PV device may be coatedwith at least one anti-reflective coating such as a thin material layeror an encapsulating film.

The steps may also comprise operations to delineate cell or modulecomponents. Delineation ordinarily comprises cutting grooves intoback-contact layer 120 to provide electrically separate back-contactcomponents. A second delineation step comprises cutting grooves,segments, or holes into at least absorber layer 130. The seconddelineation step may also comprise cutting into at least one offront-contact layer 150, buffer layer 140, or back-contact layer 120. Athird delineation step comprises cutting grooves into at leastfront-contact layer 150. The second and third delineation step may bemade simultaneously. Delineation, also called patterning, is preferablydone using at least one laser.

FIGS. 3A-3B are sputter depth profiling graphs plotting the counts ofvarious elements within the optoelectronic device versus approximatesputter depth. FIG. 3A is for a device manufactured according to theinvention and FIG. 3B is for a device manufactured according to onemethod corresponding to the best mode of carrying out the invention.FIGS. 3A-3B would require a calibration, for example based on coppercontent, to be directly compared. FIG. 3A corresponds to a PV devicesubjected to a supply comprising about 4 to 6 times more sodium thanpotassium, namely 20 min of sodium fluoride and 5 min of potassiumfluoride. Sodium and potassium may be supplied in successive, insynchronous, or in reverse steps. A person skilled in the art will knowthat the amount of alkali elements supplied does not correspond to theamount of alkali elements that remain in the absorber layer uponmanufacturing completion. Furthermore, supply of alkali elements maycontribute to the removal of alkali elements that may already be presentwithin the absorber layer. FIG. 3B corresponds to alaboratory-manufactured PV device subjected to a supply comprising aboutas much sodium as potassium, namely 20 min cumulated evaporation ofsodium fluoride and 20 min cumulated evaporation of potassium fluoride.Durations are cumulated evaporation durations. For a device that ismanufactured industrially according to the invention, the person skilledin the art may use other, preferably shorter, cumulated evaporationdurations to obtain best mode devices. An at least one alkali metal,potassium 336, is plotted with one more alkali metal, sodium 337. Ofspecial interest to characterize a device manufactured according to theinvention's best mode, independently of aforementioned manufacturingdurations, is the upper peak of potassium counts 3361 occurring at thesurface of the absorber layer. As shown in FIG. 3A, said upper peak ofpotassium counts 3361 is ordinarily not present in devices that are notmanufactured according to the best mode of carrying out the invention.Numerical analysis on various graphs may be done on locally weightedscatterplot smoothing (LOESS)-smoothed data (for example with smoothingparameter.alpha.=0.7). On the graph for potassium 336, starting from thepeak of potassium counts and progressing into the absorber layer onereaches an inflection point marking the base 3362 of the upper peak ofpotassium. The width 3363 of the base 3362 of said upper peak ofpotassium 3361 is in the range from about 0.1 .mu.m to 0.5 .mu.m,preferably from about 0.15 .mu.m to 0.3 .mu.m, more preferably about 0.2.mu.m. Said width may be interpreted as a depth into the absorber layer130. The height 3364 of said upper peak of potassium 3361, measured fromthe number of counts above its base 3362, is in the range from about 0.2to 17 times, preferably from about 0.6 to 6 times, more preferably fromabout 1.1 to 2 times the number of counts from the point of minimumnumber of counts in potassium 3365 of the absorber layer to the numberof counts at the base 3362 of said upper peak of potassium 3361. Thegraphs also present data for copper 330, representative of the absorberlayer, of zinc 350, representative of the front-contact layer, and ofmolybdenum 320, representative of the back-contact layer. Also ofguidance for manufacturing of a successful device is the ratio ofpotassium to sodium at a given point along the depth into the absorberlayer. The ratio of counts for the peak of potassium to that of the peakof sodium is in the range from about 10 to 80, preferably about 12 to50, more preferably about 16 to 20. The ratio of counts for the depth oflowest count of potassium to that of sodium at same depth is in therange from about 8 to 80, preferably about 10 to 50, more preferablyabout 18 to 22. The graphs show that, very approximately, at a givendepth, the counts of potassium are about an order of magnitude greaterthan the counts of sodium. At a given depth, counts of sodium in FIG. 3Bare also about half an order of magnitude lower than in FIG. 3A, eventhough a similar cumulated evaporation of sodium fluoride is used.Device photovoltaic conversion efficiencies are 17.8% for FIG. 3A and18.9% for FIG. 3B. Interval of layers 370 from back-contact layer 120,exclusive, to front-contact layer 150, inclusive, is measured from thelog-scale plot's half-height of shallowest maximum of back-contact layerto half-height of shallowest maximum of front-contact layer,respectively. The sputter depth profiling graph was obtained usingsecond ion mass spectrometry (SIMS). Depth profiling data were obtainedwith a SIMS system using O.sub.2.sup.+ primary ions with 2 kV ionenergy, 400 nA, and 300.times.300 .mu.m.sup.2 spot. The analyzed areawas 100.times.100 .mu.m.sup.2 using Bi.sub.1.sup.+ with 25 kV ionenergy.

FIG. 3C is an energy dispersive X-ray (EDX) line scan graph acquired ina transmission electron microscope enabling a comparison of copper toselenium content (Cu/Se) into the thickness of the absorber layer 130for two photovoltaic devices. Using Cu/Se data enables comparisonbetween devices, using selenium content as a normalizing baseline. Thedistance from the surface of the device after the step of formingcavities should here only be considered as relative and indicative. Aperson skilled in the art of EDX measurements will know that thedevice's tilt angle within the microscope will influence the graph'sscale. The plot for a first device 380 is representative of a devicewhere, prior to the step of forming cavities 236, the alkali crystalsembedded in the surface of the absorber layer do not comprise potassium.Said first device is then subject to the step of forming cavities 236and the copper content measured by EDX line scan to produce said plot.The plot for the second device 390 is representative of a device where,prior to the step of forming cavities 236, the alkali crystals embeddedin the surface of the absorber layer comprise potassium. Morespecifically, the plot for the second device 390 is for a devicecomprising sodium and potassium. Said second device is then subject tothe step of forming cavities 236 and the copper content measured by EDXline scan to produce the plot for the second device 390. The seconddevice corresponds to the best mode for carrying the invention. The EDXplot for the second device comprises an extended region of low and aboutconstant Cu/Se 391 extending from the surface of the absorber layer intoa portion of the depth of the absorber layer. Said low and aboutconstant region 391 has a depth in the range from about 0.05 .mu.m to0.5 .mu.m, preferably 0.1 .mu.m to 0.4 .mu.m, more preferably 0.2 .mu.mto 0.3 .mu.m. It is ordinarily in this region that some copper isreplaced by, for example, cadmium and/or oxidation state +1/+2 elements.The extended region 391 is said low because at depths greater than saidregion into the absorber layer, the value of Cu/Se rises and thenremains at a constant value at further depth into the absorber layer.

FIGS. 4A to 4D present data obtained for a device where the step offorming cavities 236 comprises, for a first dataset, the steps ofaqueous wetting 237 and treating absorber surface 238, and for a seconddataset, additionally comprises forming buffer layer 240. FIGS. 4A to 4Dare graphs illustrating the effect of adding an alkali metal comprisingpotassium and the effect of forming a buffer layer comprisingcadmium-sulfide on PV parameters. FIGS. 4A to 4D are drafted, forillustrative purposes representative of prior art, with a baselinemethod where an alkali metal comprising sodium is also added. Thefigures show that the greater the amount of potassium, the greater theopen circuit voltage V.sub.00, the greater the fill factor, and thegreater the PV efficiency. The figures also show that a step of forminga buffer layer contributes to a greater open circuit voltage, a greatershort circuit current, a greater fill factor, and a greater PVefficiency. A step of forming a cadmium sulfide buffer layer based on aCBD process that lasts about 5 minutes enables an increase in PVefficiency in a range from about 3% to 7%. Said step of forming bufferlayer is with a temperature of about 70.degree. C. FIGS. 4A to 4Dpresent data for a laboratory process with similar supply rates forsodium and potassium evaporation sources. The baseline evaporationduration is 20 min for sodium and a set of tests where potassiumevaporation durations are 20, 40, and 60 min. A person skilled in theart will readily reduce said durations for an industrial manufacturingsystem where supply rates will be greater, possibly different fordifferent sources of alkali metals, and enable shorter durations.Furthermore, the ratio of alkali metal supplies will advantageously beadjusted for the different alkali metals. FIGS. 4A to 4D illustratepotassium over sodium alkali metal supply ratios ranging from about 1 to3 but said ratios will preferably be in the range from 2 to 20, morepreferably 2 to 10. For example, a PV device manufactured with apotassium to sodium ratio of 3 with no step for forming buffer layerresults in a PV efficiency of about 12% with a V.sub.00 of about 0.54 V.A similar device manufactured with a step of forming buffer layer 240results in a PV efficiency of about 16.7% with a V.sub.00 of about 0.68V.

FIGS. 5A-5E are graphs derived from X-ray photoelectron spectrometry(XPS) enabling comparison of the effect of adding at least one alkalimetal 235 on the chemical composition of the absorber layer surface.Measurements are obtained after the step of forming cavities 236.

Measurements of XPS intensity (in arbitrary units) as a function ofbinding energy (in eV) for 3 types of devices are presented: deviceswhere the step of adding at least one alkali metal 235 is not performed(abbreviated Alk0), devices where said step only comprises sodium alkalimetals (abbreviated AlkNa), and devices where said step comprises sodiumand potassium alkali metals (abbreviated AlkNaK). Following the step offorming absorber layer 230 and, where applicable, the step of adding atleast one alkali metal 235, the device was subject to the step offorming cavities 236 comprising the step of treating absorber surface238. Although results presented in FIGS. 5A to 5E are for devicesmanufactured according to a step of treating absorber surface 238lasting 22 min at SATP, a person skilled in the art will notice aCd3d.sub.5/2 peak that is substantially higher for that of AlkNaK thanthat of Alk0 or AlkNa.

FIG. 5A presents XPS data in a range from about 408 eV to 402 eV forAlk0 5010, AlkNa 5110, and AlkNaK 5210. Each curve presents a peak atabout 405.1 eV corresponding to the binding energy spectra of cadmiumCd3d.sub.5/2. The graph for AlkNaK presents a Cd3d.sub.5/2 peak 5215that, based on the 405.1 eV point on a line extending from points of 408eV to 402 eV, is at least 60% greater in height, preferably in the rangeof 60% to 1000% greater, more preferably in the range of 380% to 530%greater, for the Cd3d.sub.5/2 peak of AlkNaK than for that of Alk0 orAlkNa. This suggests that the surface of the absorber layer may compriseat least one region of increased inversion doping for AlkNaK compared toAlk0 or AlkNa. Said at least one region therefore comprises a higheramount of Cd.sub.Cu, i.e. sites where cadmium has replaced copper. Thesurface of the absorber layer comprises at least one region ofcadmium-doped ABC material, or in this case ABC.sub.2 material,specifically in this example, CIGS material. Said regions ofcadmium-doped ABC material comprise at least one cavity, preferably aplurality of cavities, the surrounding of which, comprised in a radiusof about 100 nm, preferably 50 nm, more preferably 20 nm, comprisescadmium-doped ABC material.

FIG. 5B presents XPS data in a range from about 940 eV to 925 eV forAlk0 5010, AlkNa 5110, and AlkNaK 5210. Each curve presents a peak atabout 933 eV corresponding to the binding energy spectra of copperCu2p.sub.3/2. The graph for AlkNaK presents a Cu2p.sub.3/2 peak 5225that, based on the 933 eV point on a line extending from points of 940eV to 925 eV, is substantially lower than that of Alk0 or AlkNa. SaidAlkNaK peak 5225 has a height in the range from about 0.2 to 0.7 times,preferably in the range from about 0.25 to 0.4 times, more preferably inthe range from 0.3 to 0.35 times that of AlkNa and/or that of Alk0.Similarly, FIG. 5D presents XPS data in a range from about 1122 eV to1114 eV with peaks corresponding to the binding energy of Ga2p.sub.3/2.The curve for AlkNaK presents a Ga2p.sub.3/2 peak 5227 with a height inthe range from about 0.1 to 0.7 times, preferably in the range fromabout 0.2 to 0.4 times, more preferably in the range from 0.22 to 0.32times that of AlkNa and/or that of Alk0.

FIGS. 5C and 5E present XPS peaks of about the same height for AlkNaKand AlkNa for In3d.sub.5/2 (range 448 eV to 440 eV) and Se3d5 (range 60eV to 50 eV).

Of importance to summarize FIGS. 5A to 5E is the change in area underthe peaks of the AlkNaK curve with respect to the peaks of the AlkNa orAlk0 curves. This is expressed in Table 3 to reflect the percent ofchange from AlkNa or Alk0 to AlkNaK.

TABLE 3 Area change from AlkNa or AlkO to AlkNaK for element bindingspectra Area change from AlkNa or AlkO to AlkNaK (%) Range Pref. rangeMost pref. range from-to from-to from-to Cd3d_(5/2)  40-2000 200-1500 500-1000 Cu2p_(3/2) −10-−100 −40-−100 −60-−90 In3d_(5/2) −30-30 −20-20  −10-10   Ga2p_(3/2) −10-−100 −40-−100 −60-−90 Se3d5 −30-30 −20-20  −10-10  

FIGS. 6A to 6D present a scanning electron microscope (SEM) image andits subsequent processing to obtain statistical information on themodified surface of absorber layer 130.

FIG. 6A is an SEM image of the surface of absorber layer 130. The stateof the surface of absorber layer 130 corresponds to that obtained afterat least one of the steps of aqueous wetting 237 or treating absorbersurface 238. The state is ordinarily observed prior to the step offorming buffer layer 240 and more preferably prior to the step offorming front-contact layer 250. A person skilled in the art will knowthat solutions that may be used for the step of forming buffer layer 240may contribute to forming cavities 610, 612, 613. However, for example,if the buffer layer comprises cadmium sulfide and the buffer layercontributes overall thickness to the device's thin-film, the cadmiumsulfide will fill the cavities and ordinarily reduce or prevent thevisibility of cavities upon analysis with scanning electron microscopyof the surface of the absorber layer. The surface of absorber layer 130is characterized in that it comprises cavities 610, 612, 613, asdesignated in FIG. 6B. Said cavities are ordinarily not visible afterforming front-contact layer 250. A person skilled in the art will beable to obtain images over ranges of working distances from the SEM'selectron optical column, absorber surface orientations with respect tothe axis of the SEM's electron gun, electron beam energies, andmagnifications.

FIG. 6B is FIG. 6A following digital image contrast enhancement. Aperson skilled in the art may use various contrast or feature enhancingalgorithms to assist collection of statistical data. Cavities 610 areordinarily visible using microscopy following at least the step oftreating absorber surface 238. Cavities 610, 612, 613 are visible asdarker areas formed as shapes resembling at least one square. Somecavities 612, 613 can be seen as formed by aggregates of squares and mayhave rectangular shapes or more complex shapes depending on theorientation of the multiple aggregated squares. Said cavities resultfrom selectively dissolving away from the surface of absorber layer 130of at least one crystal aggregate comprising at least one alkalicrystal. Said at least one alkali crystal preferably comprisespotassium. Said alkali crystals are ordinarily formed as cubic crystalsor compounds of cubic crystals. Dissolving of said cubic crystals orcompounds of cubic crystals embedded within the surface of the absorberlayer therefore results in square-like cavities which may appear asaggregates. A white line has been drawn over the image by theinvestigator to highlight a grain facet 620 of absorber layer 130. Theinformation bar supplied by the scanning electron microscope in FIG. 6Ais cropped out.

FIG. 6C is FIG. 6B following further digital image processing. The imageis resized to a preferred width in pixels. The image is also smoothed.An image segmentation algorithm implemented in a digital computerprogram is then applied to locate, mark, and obtain statisticalinformation on cavities. A person skilled in the art of image processingand image segmentation will know that adjusting image processingparameters requires supervised tests on a few image samples of a givenmanufacturing process to obtain correct image segmentation andstatistical data. FIG. 6C shows how cavity 610 has been darkened bycavity mark 630. The plurality of cavity marks may be used to obtainstatistical information about the cavities 610, 612, 613 on the surfaceof absorber layer 130.

FIG. 6D is FIG. 6C following removal of some cavity marks. Cavity marksmay be removed in crevices at the periphery of grain facets. Oversizecavity marks may also be removed. Removal of cavity marks may be donemanually and/or automatically. Some cavity marks may for example beremoved based on thresholds on size. Other cavity marks may be removedbased on grey level intensity or other pattern recognition methods, suchas grain identification conducted at a larger scale. Areas where cavitymarks have been removed are designated by white contours 640.

As an example for FIGS. 6A to 6D, the image in FIG. 6A is obtained byplacing at least the absorber layer 130 at a working distance of about 4mm and with the surface of absorber layer 130 facing the electron gunand having an orientation that is perpendicular to the gun's axis. Theelectron beam energy is set to 5 kV. Magnification is set at 120,000. InFIG. 6B the contrast is enhanced by digital image histogramequalization. In FIG. 6C the image is resized to a width of 800 pixels.In FIG. 6C the image is smoothed by convolution with a two-dimensionalcontinuous wavelet transform based on a Gaussian wavelet that is in therange from about 5 pixels to 12 pixels wide, preferably about 5 to 10pixels wide. The image segmentation algorithm is a watershed algorithm(Beucher S. (1991) The watershed transformation applied to imagesegmentation, In 10th Pfefferkorn Conf. on Signal and Image Processingin Microscopy and Microanalysis, 16-19 Sep. 1991, Cambridge, UK,Scanning Microscopy International, suppl. 6. 1992, pp. 299-314). Thewatershed algorithm is implemented in the Gwyddion software program, afree, open source, and cross-platform software (Gwyddion 2.26, retrieved2014 Apr. 30 from <URL: www.gwyddion.net>). Parameters fed into thewatershed algorithm via the Gwyddion software are, with inverted height,for the grain location stage, a number of steps of about 20, a drop sizeof about 0.3, and a threshold of about 5 pixels squared and, for thesegmentation stage, a number of steps of 80, and a drop size of 3.0provide results in Table 3. Ranges for various watershed analysisresults that may be helpful to guide the person skilled in the arttowards manufacturing photovoltaic devices according to the inventionare provided in Table 4.

TABLE 4 Watershed image processing results for FIGS. 6A to 6D Watershedimage All marked cavities Some marks removed processing results (FIG.6C) (FIG. 6D) Image dimensions (nm)  1055 × 738.5  1055 × 738.5 Imageresolution (pixels) 800 × 560 800 × 560 Number of cavities 658 620Number of cavities per 845/μm² 796/μm² unit area Total projected area297 × 10⁻¹⁵ m² 243 × 10⁻¹⁵ m² (absolute) Total projected area 38.08%31.16% (relative) Mean cavity area 0.45 × 10⁻¹⁵ m² 0.39 × 10⁻¹⁵ m² Meancavity size 18.4 nm 17.4 nm Grain area sample variance 2.51 × 10⁻³¹ 1.36 × 10⁻³¹ 

TABLE 5 Ranges for Watershed image processing results Some marks removedWatershed image Range Pref. range More pref. range processing resultsfrom-to from-to from-to Image dimensions  1055 × 738.5  1055 × 738.5 1055 × 738.5 (nm) Image resolution 800 × 560 800 × 560 800 × 560(pixels) Number of cavities   1-3000  100-1200 200-800 Total projected15%-80% 20%-60% 24%-45% area (relative)

A person skilled in the art wishing to evaluate results derived from anapparatus manufactured according to the method, or wishing to compareimage analysis software may, for calibration purposes, use the image ofFIG. 6A or the freely available texture image 1.5.03 “Rough wall”available from the University of Southern California Signal and ImageProcessing Institute USC-SI PI Image Database, retrieved 2014 Apr. 30from <URL:http://sipi.usc.edu/database/download.php?vol=textures&img=1.5.03>.Image 1.5.03 processed using the Watershed algorithm in Gwyddion,processed in the same way and parameters as for images 6A to 6C,provides results in Table 5.

TABLE 6 Watershed image processing results for USC- SIPI image 1.5.03“Rough wall” Watershed image processing results USC-SIPI image 1.5.03Arbitrary image dimensions (mm) 100 × 100 Image resolution (pixels) 512× 512 Number of cavities 516 Total projected area (absolute) 3.44 × 10⁻³m² Total projected area (relative) 34.43% Mean cavity area 6.7 mm² Meancavity size 2.31 mm

From Watershed image segmentation on a range of samples manufacturedaccording to the present invention, a person skilled in the art mayspecify ranges that characterize the surface of absorber layer 130following at least one of bathing in diluted ammonia 237 or treatingabsorber surface 238.

FIG. 6E presents a transmission electron microscope (TEM) image of across-section of PV device detailing the area where a plurality ofcavities, three of which are indicated as 610, 612, and 613, arecomprised at least between absorber layer 130 and front-contact layer150. The image shows that cavities are visible as darker squares ofabout 5 nm or compounds of squares in the range of about 5 nm to 40 nm.

FIG. 6F presents an SEM image of the surface of absorber layer 130comprising a plurality of alkali crystals, three of which among aplurality are indicated for illustrative purposes as 660, 662, and 663.The image of FIG. 6F was obtained prior to subjecting the surface of theabsorber layer to the step of forming cavities 236. The image also showsthat said alkali crystals are aggregates of cubic crystals with widthranging from about 10 nm to 50 nm, more commonly about 20 nm. Saidalkali crystal aggregates have a greatest width ranging from about 20 nmto 200 nm, more commonly about 100 nm. Alkali crystal aggregates visiblein the image are partly embedded into the surface of the absorber layer.The visible alkali crystal aggregates 660, 662, 663, are formed as aresult of the steps of forming absorber layer 230 and adding at leastone alkali metal 235, one of said alkali metal comprising potassium. Thecomposition of the material directly in contact with the alkali crystalsmay be modified due to a higher local concentration of alkali elements.For example, the composition of the absorber layer directly in contactwith the alkali crystals may be rich in alkali elements. As a moreprecise example, if one of the at least one alkali elements supplied ispotassium, then the composition of the absorber layer directly incontact with an alkali crystal may be potassium rich. A plurality ofalkali crystals and alkali crystal aggregates are selectively dissolvedby the step of forming cavities 236. Furthermore, portions of material,for example absorber material, directly in contact with said crystal mayalso be dissolved by the step of forming cavities 236. It is alsopossible that a plurality of alkali crystals and alkali crystalaggregates that are completely embedded under the surface of theabsorber layer and therefore not visible in FIG. 6F are dissolved by thestep of forming cavities 236.

FIG. 7 is a graph of device reflectance as a function of wavelengthprior to forming a front-contact layer 250, 150. It enables a comparisonbetween a first device where adding at least one alkali metal 235 doesnot comprise potassium, thereby resulting in the first reflectance curve780 for a non-potassium comprising device, and a second device whereadding at least one alkali metal 235 comprises potassium, therebyresulting in the second reflectance curve 790 for a potassium comprisingdevice. As an example for FIG. 7, the reflectance curve for anon-potassium comprising device 780 is for a device where adding atleast one alkali metal 235 comprises adding sodium fluoride (NaF) only,and the reflectance curve for a potassium comprising device 790, a bestmode device, is for a device where adding at least one alkali metal 235comprises adding NaF and KF. The first reflectance curve shows adecreasing reflectance from about 23% at a wavelength of 300 nm to about14% at 900 nm. Said first reflectance curve also comprises at least twozones 781, 782 of about constant reflectance. A first zone 781 is atabout 18% from about 380 nm to about 440 nm and a second zone 782 atabout 15% from about 580 nm to about 800 nm. The second reflectancecurve shows a decreasing reflectance from about 11% at a wavelength of300 nm to about 7% at 550 nm, followed by an increasing reflectance toabout 12% at 900 nm. Said second curve also comprises at least two zones791, 792 of about constant reflectance. A first zone 791 is at about 18%from about 380 nm to about 440 nm and a second zone 792 at about 7% fromabout 500 nm to about 600 nm. The overall reflectance of the firstdevice is therefore lower than that of the second device.

Although the step of forming a front-contact layer 250, 150, andespecially the thickness of said layer has an effect on the reflectanceand color of a device, a person skilled in the art may wish to adjustthe amount and composition of alkali metals in the step of adding atleast one alkali metal 235 so as to contribute to the design of thedevice's optical properties or to the selection of a color for the PVdevice after the step of forming front-contact layer 250 has beencompleted. One may therefore devise a method of selecting the color of aPV device and/or its reflectance by adjusting the amount and/orcomposition of the at least one alkali metal in the step of adding theat least one alkali metal.

FIG. 8 presents an apparatus 800 for carrying out the method offabricating thin-film optoelectronic devices 100, 200. The apparatuscomprises at least one system for forming at least one absorber layerand adding at least one alkali metal 810 and at least one system forforming cavities 850. Said apparatus may also comprise other subsystems880 for completing any of the steps of short drying 242, annealing 244,degassing 246, or forming front-contact layer 250. Said apparatus 800may be a roll-to-roll system for coating of flexible substrates.

For example, the system for forming at least one absorber layer andadding at least one alkali metal 810 comprises means 820 for providing asubstrate 110 with a back-contact layer coating 120. For a roll-to-rollsystem, said means 820 for providing a substrate 814 may be a payoutreel 820. Said system coats the back-contact layer coating side of saidsubstrate with at least one absorber layer 130 made of an ABCchalcogenide material, including ABC chalcogenide material quaternary,pentanary, or multinary variations, wherein A represents elements ofgroup 11 of the periodic table of chemical elements as defined by theInternational Union of Pure and Applied Chemistry including Cu and Ag, Brepresents elements in group 13 of the periodic table including In, Ga,and Al, and C represents elements in group 16 of the periodic tableincluding S, Se, and Te. Said substrate may for example be coated by aplurality of evaporation sources 831 s providing material to thesubstrate via evaporation plumes 831 p. Said plumes may overlap on thesurface of the substrate. Said evaporation sources may be arranged inblocks 830, 835, wherein, for example, a first block 830 dispensesmaterial for the step of forming the absorption layer 130, 230 and asecond block 835 dispenses material for adding at least one alkali metal235 as the substrate moves in direction 815 over said sources. Saidsubstrate may pass through a plurality of rollers 825, such astensioning rollers, and be collected by a take-up reel 822.

The system for forming cavities 850 comprises, for example, a payoutreel 820 for providing a substrate 854 comprising an absorber layer 130.Said system comprises means for forming cavities. The system may forexample comprise at least one wetting or bathing device 855 for at leastone of the steps of forming cavities 236, namely aqueous wetting 237,treating absorber layer surface 238, or forming a buffer layer 240. Anadvantageous solution may be for the system to comprise a plurality ofwetting devices 855, for example waterfall wetting devices, showeringdevices, spraying devices, humidifying devices, or bathing devices,thereby enabling a plurality of steps for forming cavities within thesame system. A person skilled in the art may also devise an apparatuswhere the substrate continuously transfers, for example withoutintermediate take-up reels, from a first system to a second system.

What is claimed is:
 1. A thin-film optoelectronic device, comprising: asubstrate; a back-contact layer; an absorber layer formed of an ABCchalcogenide material, wherein A represents one or more elementsselected from a group consisting of copper (Cu) and silver (Ag), Brepresents one or more elements selected from a group consisting ofindium (In), gallium (Ga), and aluminum (Al), and C represents one ormore elements selected from a group consisting of sulfur (S), selenium(Se), and tellurium (Te); at least one alkali metal; and at least onecavity at a surface of the absorber layer formed by dissolving at leastone crystal aggregate away from the surface of the absorber layer,wherein the at least one crystal aggregate comprises at least one alkalicrystal comprising the at least one alkali metal.
 2. The device of claim1, wherein the at least one alkali metal comprises potassium (K).
 3. Thedevice of claim 1, wherein the at least one alkali crystal comprises acubic crystal.
 4. The device of claim 1, wherein the at least one alkalimetal comprises potassium and wherein a curve for counts of potassium inthe device's sputter profiling graph comprises an upper peak ofpotassium within a depth ranging from the surface of the absorber layerto about 0.5 μm into the absorber layer.
 5. The device of claim 4,wherein the upper peak of potassium comprises a base having a width in arange from about 0.1 μm to about 0.5 μm.
 6. The device of claim 4,wherein the upper peak of potassium has a height measured from thenumber of counts above its base that is in a range from about 0.2 toabout 17 times the number of counts from the point of minimum number ofcounts in potassium of the absorber layer to the number of counts at thebase of the upper peak of potassium.
 7. The device of claim 1, whereinthe at least one alkali metal comprises potassium, a curve for a copperto selenium content in the device's energy dispersive X-ray line scangraph comprises an extended region of low and about constant Cu/Seextending from the surface of the absorber layer into a portion of thedepth of the absorber layer, and the extended region of low and aboutconstant Cu/Se has a depth in the range from about 0.05 μm to about 0.5μm.
 8. The device of claim 1, wherein the at least one alkali metalcomprises potassium, the device comprises +1/+2 elements, and the X-rayphotoelectron spectrometry curve comprises a cadmium (Cd) 3d^(5/2) peakthat is at least about 60% greater in height than for the curve of adevice wherein the at least one alkali metal does not comprisepotassium.
 9. The device of claim 1, wherein the at least one cavity atthe surface of the absorber layer comprises a plurality of cavities, thecavities covering a total relative projected area in a range from about15% to about 80% of the surface of the absorber layer.
 10. The device ofclaim 1, wherein the at least one cavity at the surface of the absorberlayer comprises a plurality of cavities, the cavities having a meancavity area in a range from about 0.1×10⁻¹⁵ m² to about 0.8×10⁻¹⁵ m².11. The device of claim 1, wherein the at least one cavity comprisescadmium.
 12. The device of claim 1, further comprising at least onefront-contact layer.
 13. A thin-film optoelectronic device, comprising:a substrate; a back-contact layer; an absorber layer formed of an ABCchalcogenide material, wherein A represents one or more elementsselected from a group consisting of copper (Cu) and silver (Ag), Brepresents one or more elements selected from a group consisting ofindium (In), gallium (Ga), and aluminum (Al), and C represents one ormore elements selected from a group consisting of sulfur (S), selenium(Se), and tellurium (Te); at least one alkali metal; and at least onecavity at a surface of the absorber layer formed by dissolving at leastone crystal aggregate away from the surface of the absorber layer,wherein the at least one crystal aggregate comprises at least one alkalicrystal comprising the at least one alkali metal, and the at least onecavity comprises cadmium (Cd).
 14. The device of claim 13, wherein theat least one alkali metal comprises potassium, the device comprises+1/+2 elements, and the X-ray photoelectron spectrometry curve comprisesa Cd 3d^(5/2) peak that is at least about 60% greater in height than forthe curve of a device wherein the at least one alkali metal does notcomprise potassium.
 15. The device of claim 13, wherein the at least onecavity at the surface of the absorber layer comprises a plurality ofcavities, the cavities covering a total relative projected area in arange from about 15% to about 80% of the surface of the absorber layer.16. The device of claim 13, wherein the at least one cavity at thesurface of the absorber layer comprises a plurality of cavities, thecavities having a mean cavity area in a range from about 0.1×10⁻¹⁵ m² toabout 0.8×10⁻¹⁵ m².
 17. A thin-film optoelectronic device, comprising: asubstrate; a back-contact layer; an absorber layer formed of an ABCchalcogenide material, wherein A represents one or more elementsselected from a group consisting of copper (Cu) and silver (Ag), Brepresents one or more elements selected from a group consisting ofindium (In), gallium (Ga), and aluminum (Al), and C represents one ormore elements selected from a group consisting of sulfur (S), selenium(Se), and tellurium (Te); at least one alkali metal comprising potassium(K); and at least one cavity at a surface of the absorber layer formedby dissolving at least one crystal aggregate away from the surface ofthe absorber layer, wherein the at least one crystal aggregate comprisesat least one alkali crystal comprising the at least one alkali metal.18. The device of claim 17, wherein a curve for counts of potassium inthe device's sputter profiling graph comprises an upper peak ofpotassium within a depth ranging from the surface of the absorber layerto about 0.5 μm into the absorber layer.
 19. The device of claim 18,wherein the upper peak of potassium comprises a base having a width in arange from about 0.1 μm to about 0.5 μm.
 20. The device of claim 18,wherein the upper peak of potassium has a height measured from thenumber of counts above its base that is in a range from about 0.2 toabout 17 times the number of counts from the point of minimum number ofcounts in potassium of the absorber layer to the number of counts at thebase of the upper peak of potassium.